The present invention relates to a DC battery charger, and more particularly to a charging circuit that continuously supplies unfiltered charging current to the battery with brief high frequency, high amplitude discharging current pulses superimposed on the charging current wave form.
As is well known, continuous application of a charging current to a DC battery initially is very efficient, with the supplied energy generating the proper chemical changes within the battery. However as charging continues, the efficiency eventually begins to decrease due to the increasing internal resistance of the battery. If charging current alone is maintained, eventually the plates will overheat and warp, causing the battery to short out internally.
These problems have been addressed by various elaborate methods, usually including the application to the battery of a discharge current of considerable duration, while the charging current is interrupted. For example, Tichenor, U.S. Pat. No. 2,503,179, requires interruption of the charging current before application of a discharging current to the battery. One significant disadvantage of these schemes, is the necessity for a heavy duty switch capable of repeatedly interrupting the charging current.
Another significant disadvantage has been the elaborate control schemes employed to determine when the charging current should be interrupted. For example Mas, U.S. Pat. No. 3,816,806, requires the battery to have an opening to the atmosphere for monitoring the rate of gas evolution within the battery. Then the charging current is regulated accordingly. Such physical intrusion into the battery is not only time-consuming, but on a regular basis may greatly increase the opportunity for the introduction of contaminants from the environment into the battery.
Another disadvantage has been the necessity of storing the discharge current for later introduction into the battery during the charging portion of the cycle, to maintain charger efficiency. For example, Steigerwald, U.S. Pat. No. 4,211,969, utilizes a storage capacitor 18 to store the discharge energy and two chopper circuits to control the system.
Of significant disadvantage is the relatively long duration of the discharge pulse in the prior battery systems. This long duration requires excessively heavy electrical components, with a large cross sectional area, capable of carrying the large current and dissipating the heat generated by the resistive losses of the components (I.sup.2 R losses).
Some battery charging systems compound the heating problem by increasing the duration of discharge pulses after the battery voltage has risen above a given value, approaching a full charge status. For example DuPuy, U.S. Pat. No. 3,617,851, teaches that at 90% of full charge, the charge duration would be 1 second and the discharge duration 10 seconds. The discharge duration then increases to 60 seconds at the fully charged state.
Present battery charging systems utilize the discharging current merely for the chemical reactions that it causes. For example, Tichenor, U.S. Pat. No. 2,503,179, merely teaches that the discharge current removes some of the redeposited ions to attain a smooth, dense deposit on the battery electrodes or battery plates.